Apparatus for dielectric heating



April 14, 1964 A. R. OJELID APPARATUS FOR DIELECTRIC HEATING 3 Sheets-Sheet 1 Filed March 1, 1961 April 14, 1964 G. A. R. OJELID 3,129,312

APPARATUS FOR DIELECTRIC HEATING Filed March 1, 1961 s Sheets-Sheet 2 April 14, 1964 G- A. R. OJELID 3,129,312

APPARATUS FOR DIELECTRIC HEATING Filed March 1., 1961 3 Sheets-Sheet 3 United States Patent 3,129,312 APPARATUS FOR DIELECTRIC HEATING Giiran Axel Rune Ojelid, Huskvarna, Sweden, assiguor to Husqvarna Vapenfahriks Alrtiebolag, Huskvarna, Sweden Filed Mar. 1, 1961, Ser. No. 92,688 Claims priority, application gweden d, 1960 t Claims. (Cl. 219-4055) This invention relates to an apparatus for dielectric heating, particularly of food, by means of short, electromagnetic waves, that is, micro-waves, in a closeable, oven space or mufl'le of metal connected to a high frequency generator receiving its supply of current from an electric supply or distribution system.

Ovens of this kind comprise, in the prior art, a heavy and bulky equipment of inductive components, such as transformers and inductors adapted to supply high-voltage direct current to a high frequency generator connected to the treatment chamber of the oven. The equipment in question thereby forms a unit connected to the generator by means of conductors for the high-voltage direct current and wires for filament current, and the generator is also provided with cooling means in the form of a water jacket or flanges and fan.

It is also possible to provide devices of this kind with a closeable treatment chamber of sheet iron which is in ohmic connection with a grounded output terminal of the high frequency generator fed with electric power through a transformer or another non-ohmic connection with the electric power supply system. One object of the invention is to makepossible ran ohmic connection between the high frequency generator and the electric power supply system.

Another object of the invention is to reduce the volume of the total electric equipment of the oven from an order of 1-2 times the normal volume of the corresponding treatment chamber to approximately a quarter of the same volume.

With these and other objects in view the generator is, according to the invention, connected to the main electric supply system by means of an ohmic connection com prising a transforming, converting or transducing device adapted to increase the voltage applied by the supply system.

Further objects and advantages of the invention will become more fully apparent from the following description and the accompanying drawings illustrating some preferred embodiments of the invention, and in which:

FIGURE 1 is a plan view, partly in section, of a microwave oven according to a first embodiment of the invention;

FIGURE 2 is a side view of the oven according to FIG- URE 1;

FIGURE 3 illustrates, in substantially the same way as FIGURE 1, another embodiment of the invention;

FIGURE 4 is a perspective view of a third embodiment of the apparatus according to the invention;

FIGURE 5 is a circuit diagram for the embodiment illustrated in FIGURE 4;

FIGURE 6 illustrates a modification of the apparatus according to the invention and shows an oven muffle and a unit substantially according to FIGURE 4 as viewed from behind.

The apparatus according to FIGURES 1 and 2 comprises a treatment chamber 1 of sheet aluminum or the like which carries, at its rear wall, a high frequency generator in the form of an air cooled magnetron 2. for the frequency 2450 mc., a cooling fan or blower 3 therefor, and a transforming unit or converter 4 for electric current taken directly from the main electric supply system via a connection 5. The electric supply system may be a direct current system or an alternating current system for the frequency 50-60. cycles. The purpose of the unit 4 is to increase and possibly also rectify the voltage of the electric supply system, so that this increased voltage can serve as anode voltage for the magnetron 2, to which the unit is connected by means of two conductors 6. Within the unit 4- there is at least in one direction an ohmic or galvanic connection between the electric supply system connection 5 and the conductors 6. The unit 4 may for instance "be of the type called voltage doubling rectifier. One of the conductors "6 is also, via the magnetron 2, in ohmic connection with the treatment chamber or muflie 1. As a protection against manual touch these parts connected to the electric supply system are encased in a protective housing 8 preferably of sheet metal. The housing 8 is effectively insulated from the chamber 1 and the parts mounted thereon, in that the housing supports the treatment chamber solely by means of a front member 9 of well insulating material, such as polyester lamina, mounted on the front side of the chamber 1. A door 10 located outside the front member 9 is adapted to shut the treatment chamber 1 without being in ohmic connection therewith. This is obtained substantially in that the outer sheet metal portion 11 of the door extends a quarter of a wave-length outside the opening 12, through which access may be had to the chamber, thus constituting a cavity with a depth of a quarter of a wave-length around the opening .1 2. The sheet metal portion 11 of the door as well as an internal concave (as seen from the interior of the oven) sheet metal portion 13 of the door are isolated from the treatment chamber through an adequate inter space, and the door 10 has a protective ground connection together with the housing 8. Touch protection for the Walls of the chamber 1, even when door 10 is open, is provided by a switch 15'- actuated by the door 10. This switch which is necessary as a protection against high frequency radiation is adapted to interrupt all connections between the unit 4 and the electric power supply system and in addition to that ground the chamber 1 when the door 19 is opened. In addition to that the mechanical connection between the switch 15 and the door 10 is positive, which means that the door cannot be opened if the switch has not been set in its protective position.

In the embodiment according to FIGURE 3 the door 14 of the oven has an ohmic connection through its hinges with the muffie or oven chamber 1 which in its turn is grounded together with the protective housing 16. The magnetron 2a is, however, galvanically or ohmicly insulated from the mufile 1 by means of a device comprising a Washer 17 of Teflon or a similar material (with a low loss-factor at ultra high frequency) between two flanges 18 and 19 at the high-frequency output of the magnetron. The circuits for low-frequency alternating current and direct current are entirely isolated from the mufile 1 by this device. This permits utilizing a rectifier network 21 diagrammatically shown in FIGURE 3 which in a manner known per se doubles the peak value of the alternating voltage applied and thereby replaces a heavy and bulky transformer equipment. As in FIGURES 1 and 2 the magnetron is supported by two brackets the upper one 22 of which is partly broken away in FIGURE 3.

The apparatus or oven for dielectric heating shown in FIGURE 4 is adapted to be built in and comprises a treatment chamber or muffie 1a of sheet metal which is united with a front member 24 ofsheet metal or wood in which a door (not shown) is arranged in front of the chamber 1a. At one side of the treatment chamber 1a the entire electric equipment of the oven is arranged in the form of a unit which is secured by means of screws 26 to angular brackets 27 or the like on the Wall of the chamber 1a. In front of the unit the front member 24 is perforated or provided, in another way, with openings 28 with a total area of at least 100 cm? for the circulation of air. In the same portion of the front member 24 there is also provided an elongated opening for a bank of setting keys 29 or push-buttons. These keys 29 are adapted to actuate a multi-polar switch 31 which is rigidly secured to the unit and constitutes its foremost portion. Somewhat behind and laterally of the switch 31 a rectifier assembly 32 is arranged in the unit. This assembly comprises six electrolytic capacitors 33 projecting forwards from an insulating plate 34 and twelve silicium diodes 36 mounted in holders behind the plate. The plate 34 is provided with rather big apertures 37 which permit, in

conjunction with slots 38 at the sides of the plate, air circulation around the capacitors 33 and the diodes 36. Beside the rectifier assembly 32 a filament current transformer 39 for about 10 va. is mounted. Next behind the rectifier assembly a tangent flow blower 41 with two impellers 42, 43, and driving motor 44 is mounted on a vertical shaft. The impellers have such a shape and rotation direction, that air is sucked in at the circumference of the wheels in a manner know per se and is exhausted backwards through two rectangular openings 47. In order to prevent recirculation through the blower 41 screening plates 48 and 49 are provided inwardly and outwardly respectively of the same. The rearmost part of the unit is constituted by the high-frequency generator 51, preferably a magnetron operating in the frequency band 2400-2500 me. The generator 51 is provided with cooling flanges of sheet copper which are soldered into the anode block of the magnetron and form two cell pockets 53 located exactly before the exhaust openings 47 of the blower. At the output side of the generator 51 a relatively short piece of a wave guide 54 is connected by means of strong fittings (not shown). The wave guide 54 is terminated, at a distance equal to an even multiple of a quarter of the wave length therein, by means of a small flange 56 which permits a capacitive coupling or connection of the treatment chamber la, which also contains a continuation 57 of the wave guide 54. The electromagnetic waves radiated intothe treatment chamber 1a are preferably horizontally polarized. Said distance is counted from the wave input, that is the axis 60 of the calotte 58 of the wave guide 54, to the flange 56. There is no ohmic connection at the flange 56 between the wave guide 54 and its continuation or to the chamber la.

The entire unit is held together by two rigid Bakelite gables 59 which substantially directly support the individual parts of the unit and besides permit a completely insulated but nevertheless stable attachment of the unit to the abovementionned angular brackets 27 on the wall of the treatment chamber In. With regard to the fact that the generator 51 of this device is in ohmic connection with the electric supply system terminals said insulating construction and said connection constitute very important features.

The operation of the device is best understood from FIGURE which also illustrates the details of the switch 31. Switch 31 is specific thereby, that it connects the rectifier assembly 32 with a constant switching in delay and an adjustable switching oif delay. For this purpose it is provided with a RC-circuit comprising a plurality of resistors 60, 61, 62, 63, 64, 65, 66 and a capacitor 67, as well as a rectifier 68 with a filter capacitor 69 in addition to a potentiometer 71 and two thyratrons 72, 73 which are coupled to a network of resistors 75, 76, 77, 78, 79 and adapted to control a relay 81. The switch 31 has ten control keys or push-buttons. Of these keys seven individual ones are adapted for the selection of the switching off delay, one, 80, is adapted to apply start impulses, and the two remaining push-buttons govern a pilot switch 82. The mode of operation is as follows:

When the pilot switch 82 is open, current is applied only to the transformer 39, and this current passes through a resistor 83 or a diode substituted therefore which consumes only little power but limits the supply of filament current from the'transforrner 39 to the generator 51, so that the cathode 50 thereof is kept at a slightly elevated readiness temperature without being consumed. When the switch 82 is closed, the resistor 83 is by-passed, so that the filament current is greatly increased at the same time as charging of the capacitor 67 by the rectifier 68 through the resistor 66 is initiated. After an interval of e.g. ten seconds the capacitor 67. has been charged to such 'a voltage, that the thyratron 73 is fired and in its turn immediately fires the thyratron 72 by rising its anodecathode-voltage. The network of resistors 75-79 is so dimensioned, that the current through the operating coil of the relay 81 does not reach its operating value during this sequence of events. Those ten seconds during which the relay 81 is thus not operative correspond to the time necessary for rising, bymeans of the greater filament current, the cathode 50 to its operation temperature. After that, 'a suitable operation time for the oven may be selected by manipulating one or more keys for the switching-off delay of the switch 31. At the manipulation instance nothing occurs that may be observed, since the thyratron 73 is still conducting, but when the start impulse key is manipulated, the thyratron 73 is extinguished, thus incurring an unbalance in the network 75-79 which causes full operating current through the operating coil of the relay 81. This means that the electric supply system will be connected to the rectifier assembly at the same time as the by-pass of the resistor 83 is opened. In the rectifier portion a rectification of the alternating current of the electric supply system takes place at the same time as its peak voltage is raised 2 times, since the rectifiers 36 and the capacitors 33 form, in a manner known per se, two cascaded voltage doubling stages. This means that the rectifier assembly 32 can yield at 380 v. from the electric supply system, a direct voltage of 380 /2 2 =22OO volts, which is sufiicient to make a high frequency generator of the magnetron type furnish a high frequency output of at least 1 kw. This necessitates, however, a supply of almost 2 kw. direct current power from the rectifier assembly, which is not possible, with the use of conventional components, without an exceedingly efficient cooling of the capacitors 33 as well as the rectifiers 36. According to the invention such cooling is provided thereby, that fresh air sucked in by the blower 41 through the openings 28 of the front member 24 of the oven (FIGURE 4) is caused to pass through the openings 37 and the slots 38 and in so doing sweeps.

over the capacitors 33 and the rectifying diodes 36. By the compact design of the unit and by the use of a tangential flow blower the cooling is, in addition to that, obtained without extra costs since the generator 5 1 pevertheless must be cooled. The cooling "of the generator takes place in the cell packets 53 by means of the fresh air heated only in part during its passage through the rectifier assembly, and the cooling is effective already at a power of about 20 watts of the motor of theblower.

It is also within the scope of the invention to build the oven in accordance with FIGURE 6 which illustrates a modification of the apparatus shown in FIGURE 4 and in which the treatment chamber also contains tube elements heated by means of resistors and adapted for grilling or the like, so that the walls of the treatment chamber attain a relatively high temperature. The unit is, however, of the same design but located at a larger distance'from the treatment chamber in and secured to a frame 8a common to the chamber and the unit. The frame may be made as a protective housing of metal, the gables or brackets then being insulating and the supports 87 possibly being made of metal. Inorder to avoid heat transfer in the wave guide 54, 57 the parts 54 and 57 thereof are separated by a circumferential interspace through which cooling air may be blown in order to reduce the temperature still further.

While the invention has been particularly shown and described with reference to some preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention. For example, the apparatus according to the invention may be utilized for the heating of moulding compounds or the like.

What I claim is:

1. In an apparatus for dielectric heating, by means of micro-waves, metal means forming an oven space, a magnetron type micro-Wave generator, a transmission line for micro-waves coupling said micro-wave generator to said oven space and comprising a capacitive member constituting a galvanic insulation between said oven space and said micro-wave generator; a voltage converter coupled to said micro-wave generator and to a main electric supply system, said converter comprising a galvanic connection interconnecting said micro-wave generator and said electric supply system; a grounded, protective casing for said micro-wave generator and said voltage converter, cooling means for said micro-wave generator; and supporting means carrying said micro-wave generator and insulating it galvanically from said protective casing.

2. Apparatus as claimed in claim 1, in which said voltage converter is a voltage multiplying assembly comprising capacitors and rectifiers, said assembly being also completely insulated from said oven space and from the protective casing and being located at the suction side of at least one blower for blowing cooling air over said generator and thus constituting said cooling means.

3. In an apparatus for dielectric heating, by means of micro-waves, metal means forming an oven space; a magnetron type micro-wave generator; a transmission line for micro-waves, coupling said micro-wave generator to said oven space and comprising a capacitive member constituting a galvanic insulation between said oven space and said micro-wave generator; a voltage multiplying assembly comprising capacitors and rectifiers and constituting a galvanic connection interconnecting said micro-wave generator and a main electric supply system, means completely insulating said voltage multiplying assembly from said oven space; a grounded, protective casing for said micro-wave generator and said voltage multiplying assembly; supporting means carrying said micro-wave generator and said voltage multiplying assembly and insulating them galvanically from said protective casing; and at least one blower located in said casing for cooling said voltage multiplying assembly and said micro-wave generator.

4. In an apparatus for dielectric heating, by means of micro-wave: metal means forming an oven space; a magnetron type micro-wave generator; a transmission line for micro-waves; coupling said micro-wave generator to said oven space and comprising a capacitive member constituting a galvanic insulation between said oven space and said micro-wave generator; a voltage multiplying assembly comprising capacitors and rectifiers and galvanically interconnecting said micro-wave generator and said electric supply system, means completely insulating said assembly from said oven space; a grounded, protective casing for said micro-wave generator and said voltage multiplying assembly, said casing being completely insulated from said micro-wave generator and said voltage multiplying assembly; at least one blower for blowing cooling air over said micro-wave generator and having its suction side located adjacent to said voltage multiplying assembly for drawing air through said assembly; said micro-wave generator and said voltage multiplying assembly being lo cated between and secured to two walls of electrically in sulating material, confining, together with an adjacent wall of said oven space and an adjacent wall of said protective casing, an air duct extending from the front side of said apparatus towards the rear side thereof.

References Cited in the file of this patent UNITED STATES PATENTS 2,473,834 Tuller June 21, 1949 2,627,571 Hichle et a1. Feb. 3, 1953 2,628,278 Zaleski Feb. 10, 1953 2,632,090 Ravercomb et a1. Mar. 17, 1953 2,780,705 Siegmeth Feb. 5, 1957 2,831,952 Warner Apr. 22, 1958 2,972,084 Esterson et a1. Feb. 14, 1961 

1. IN AN APPARATUS FOR DIELECTRIC HEATING, BY MEANS OF MICRO-WAVES, METAL MEANS FORMING AN OVEN SPACE, A MAGNETRON TYPE MICRO-WAVE GENERATOR, A TRANSMISSION LINE FOR MICRO-WAVES COUPLING SAID MICRO-WAVE GENERATOR TO SAID OVEN SPACE AND COMPRISING A CAPACITIVE MEMBER CONSTITUTING A GALVANIC INSULATION BETWEEN SAID OVEN SPACE AND SAID MICRO-WAVE GENERATOR; A VOLTAGE CONVERTER COUPLED TO SAID MICRO-WAVE GENERATOR AND TO A MAIN ELECTRIC SUPPLY SYSTEM, SAID CONVERTER COMPRISING A GALVANIC CONNECTION INTERCONNECTING SAID MICRO-WAVE GENERATOR AND 